1. Field of the Invention
The oxidative coupling reaction of 2,6-di-orthosubstituted phenols results in solutions of polyphenylene ethers (PPEs) in organic solvents. The concentration of the solution obtained varies to a certain degree, depending on the composition of the reaction mixture and on the reaction conditions.
2. Discussion of the Background
According to the current state of the art, PPE polymers are no longer precipitated out by adding a so-called anti-solvent, but are recovered by concentrating the solution. Other polymers, e.g., polystyrenes, and additives, may be optionally added in this step. It is difficult to carry out the concentration process because of the difficulty of removing residual amounts of solvents from the highly viscous solutions. If high temperatures are applied over fairly long periods, undesirable product damage occurs.
Indeed, the reasons for producing PPE solutions of constant concentration is to avoid having to subject the polymer to excessive thermal stressing in subsequent processing steps.
Various methods are known for concentrating solutions of organic plastics. For example, a flash evaporation stage may be provided and can be operated as a direct flow-through operation or in recycle. The direct or single-pass operation has the disadvantage that it accentuates variations in the concentration of the initial solution which are reflected in the concentrated solutions produced. The recycle operation requires temperatures which exceed the boiling point of the solvent at normal pressure by more than 40.degree. C., due to the high viscosity of the PPE solutions. The long residence time of the product in the evaporator is also disadvantageous.
In German OS No. 31 31 440, a method is described in which other polymers, e.g., styrene polymers, are added to solutions of PPE, and the resulting mixtures are evaporated in a multistage process. A disadvantage of this process is that substantial amounts of solvent must be removed from the polymer mixture, thereby requiring apparatus of large dimensions.
The methods of German OS No. 33 15 802 and OS No. 33 15 803, which are improvements over the method of German OS No. 31 31 440, comprise concentrating dilute PPE solutions to at least 60% in an evaporation process, and then processing the concentrate further with addition of plastic additives and/or melts of other polymers. It is emphasized (see OS No. 33 15 802, page 8, line 25) that it is advantageous to carry out the first evaporation process in a single stage, regardless of whether the PPE solutions:
(i) are heated under pressure in a heat exchanger and then are depressurized;
(ii) are fed to a zonewise heatable dynamic thin layer evaporator;
(iii) are concentrated in a heatable tube assembly evaporator; or
(iv) are concentrated in a zonewise heatable falling film evaporator.
All the methods described above have the disadvantage that they either expose the PPE solution to excess thermal stressing or they produce substantial variations in the concentrations of the concentrated PPE solutions when presented with variations in concentration of the dilute starting solutions.